Recently, review of refrigerants is progressing in the fields of refrigeration and air conditioning from the viewpoint of preventing global warming. In the field of car air conditioners, refrigerants with a global warming potential (GWP) of 150 or more are regulated by the EU F-Gas Regulation. Then 2,3,3,3-tetrafluoropropene (HFO-1234yf, CF3CF═CH2) that has a GWP of 4 has been used.
As for fixed-type refrigeration and air-conditioning systems, currently used refrigerants, such as R-410A (GWP: 2,088), R-404A (GWP: 3,922), R-407C (GWP: 1,770), and 1,1,1,2-tetrafluoroethane (hereinafter, HFC-134a) (GWP: 1,430), have high GWP and are thus being regulated in developed countries from the viewpoint of not only cutting CO2, but also reducing HFC (hydrofluorocarbons or fluorinated hydrocarbons). The development of alternative refrigerants is an urgent issue. Refrigerants should be selected from various refrigerants, taking into consideration the application, operating conditions, and other conditions, from multiple viewpoints, including environmental friendliness, safety, performance, and economic efficiency. Various types of refrigerants are currently proposed, together with fluorocarbon and natural refrigerants; however, currently no refrigerants satisfy all requirements, including flammability, efficiency, and GWP value. It is necessary to select the right refrigerant for the right place, depending on the application, operating conditions, and other conditions.
Among refrigerants, HFO-1234yf refrigerants have attracted attention because of their low GWP and low toxicity in fields other than the field of car air conditioners. HFO-1234yf refrigerants are a major candidate for an alternative to HFC-134a refrigerants in the field of large refrigeration and air-conditioning systems. However, as alternatives to R-410A and other refrigerants for use, for example, in fixed-type refrigeration and air-conditioning systems, HFO refrigerants alone have low vapor pressure and raise concerns of insufficient capability or performance degradation, as compared to conventional refrigerants. In addition, HFO refrigerants are known to be slightly flammable.
Accordingly, non-azeotropic refrigerant mixtures of various refrigerants have been proposed recently to improve performance and achieve non-flammability (PTL 1 to PTL 3).
However, many of the mixtures of HFC and HFO-1234yf are non-azeotropic mixtures and therefore undergo composition changes during phase changes, such as evaporation and condensation. This is because low-boiling-point components are more likely to be evaporated, and high-boiling-point components are more likely to be condensed. This tendency is prominent in the case of evaporation, i.e., a phase change from liquid to vapor, and is particularly remarkable when the components of the mixture have a large difference in their boiling points. For this reason, when such a non-azeotropic mixture is transferred from a container to another container, the mixture is usually extracted from the liquid phase so as not to induce phase changes.
Nevertheless, a mixture of components that have a large difference in boiling points undergoes a composition change of a few percent, even when the mixture is extracted from the liquid phase. This is because the reduced pressure and the increased gas phase space due to the extraction of the mixture lead to evaporation of low-boiling-point components in the liquid phase. A composition change of a few percent not only causes a significant change in refrigerant performance to thereby reduce capability and efficiency, but also has a major impact on the safety of the refrigerant, such as on flammability (PTL 4 and PTL 5).
In particular, difluoromethane (HFC-32, C2F2), which is likely to be used as a refrigerant mixture with HFO-1234yf, has a very high refrigerating capacity; however, the difference in boiling point between HFC-32 and HFO-1234yf is at least 20 K. Composition changes that occur during the transfer of such a refrigerant mixture from a feeding container (e.g., a gas cylinder or tank truck) to a refrigeration and air-conditioning system or other tanks are at a non-negligible level in terms of performance. Moreover, in terms of not only performance, but also quality assurance of the refrigerant mixture, it is important to control composition changes within the set tolerance of the refrigerant mixture.
For example, when a refrigerant mixture comprising HFO-1234yf and HFC-32 is transferred at 40° C., without taking any measures, a composition gap of up to 3 to 4 wt % from the target composition develops when the entire liquid before transfer is extracted. In this case, the composition change rate is about ±4 wt % from the target composition, and refrigeration capacity and refrigerant capacity (e.g., COP) expected from the target composition cannot be ensured. Therefore, it is important to control the composition change rate within a range as narrow as possible.
Furthermore, composition changes significantly vary depending on the type and composition ratio of non-azeotrope refrigerant, and it is difficult to predict the range of composition changes without actual measurement.